1. Field of the Invention
The present invention relates to electronical and electronic components, circuits and systems. More specifically, the present invention relates to analog to digital converters.
2. Description of the Related Art
The function of an analog to digital converter (ADC) is to accurately convert an analog input signal into a digital output represented by a coded array of binary bits. The output bits are generated by processing the analog input signal through a number of comparator steps. There are several types of ADC architectures, each architecture having different characteristics, such as bandwidth, speed, power, and resolution. A flash ADC, for example, produces an N-bit digital output in one step with 2N−1 parallel comparators. Flash ADCs provide higher speed of conversion, but are limited by higher input capacitance, power consumption, and device yield constraints associated with the high number of comparators in the circuitry. At the other extreme, a successive approximation ADC produces an N-bit digital output in N sequential steps using a single comparator. Successive approximation ADCs are simple in structure, and may be very accurate, but they have very slow conversion times due to the serial nature of the conversion process.
Subranging ADCs provide an intermediate compromise between flash ADCs and successive approximation ADCs. Subranging ADCs typically use a low resolution flash quantizer during a first or coarse pass to convert the analog input signal into the most significant bits (MSB) of its digital value. A digital to analog converter (DAC) then generates an analog version of the MSB word, which is subtracted from the input signal at a summing node to produce a residue or residual signal. The residue signal is sent through one or more fine passes (through the same quantizer or additional low resolution quantizers) to produce the lower significant bits of the input signal. The lower significant bits and the MSB word are then combined by digital error correcting circuitry to produce the desired digital output word.
Certain applications, particularly in the military communications market, require ADCs that can operate over a wide range of bandwidth, dynamic range, and power consumption. A number of new military systems want to combine several services, such as GPS data links, electronic warfare, and narrowband as well as wideband communications, into a single device. Each of these services uses a different type of waveform. Currently available ADCs, however, are not capable of adapting to different types of signals. Conventional ADCs work optimally only at a narrow range of bandwidth, resolution, and power. For instance, one ADC may operate at low resolution and wide bandwidth, while another ADC operates at high resolution and narrow bandwidth. Currently, in order for a system handle multiple services, it would require multiple ADCs to be switched in, resulting in a system that is large, heavy, expensive, and high power.
Hence, there is a need in the art for a single ADC that can operate over a wide range of bandwidth, resolution, and power consumption.